The fused rhombic lips assume a dumbbell shape - the central unpaired part is the vermis while the lateral knobs are the hemispheres. By the end of the 4th month growth causes fissures on the surface. The first fissure to develop is the posterolateral fissure which separates the flocculonodular lobe from the corpus cerebelli which rostral to it. The corpus cerebelli grows more rapidly than the flocculonodular lobes and becomes divided by the primary fissure into the anterior and posterior lobes. The surface of the lobes are further marked by the development of many closely packed transverse gyri called folia.

Histogenesis of the cerebellar cortex - In the 2nd month of development the cerebellum consists of a ventricular (inner germinal layer), mantle, and marginal zones. By 19 weeks neuroblasts dividing in inner germinal layer migrate to the marginal layer, proliferate and form the external granular (germinal) layer. The outer germinal layer produces basket, granule and stellate cells. The inner germinal layer produces the Purkinje and Golgi cells and the cells of the deep cerebellar nuclei (dentate, globose, emboliform and fastigii). Radial glial cells extend from the ventricular layer to the surface of the marginal layer and guide the migration of the developing neurons. Neuroblasts of both dividing cell layers produce glia.